1. Field of the Invention
This invention relates generally to antennas, and more particularly to antennas that are adapted for use with electronic article surveillance systems. In such systems the articles being protected are tagged with a tag containing a resonant circuit. A swept frequency interrogation transmitter whose frequency is swept through the resonant frequency of the resonant circuit contained in a tag has its output coupled to an antenna located near an exit from the protected area. A second antenna is disposed near the transmitting antenna. The second antenna is coupled to a receiver that detects a signal radiated by the tag whenever the transmitter frequency passes through the resonant frequency of the tag.
2. Description of the Prior Art
Various antennas usable for electronic article surveillance purposes are known. One such antenna is disclosed in U.S. Pat. No. 4,251,808. This patent discloses a twisted loop shielded antenna that employs a twisted loop having two or more sections shielded by a metal tube. The multiple loop sections are twisted so that they are in phase opposition, thereby confining the transmitted signal to an area close to the transmitter, and reducing the amount of signal radiated to areas outside the immediate vicinity of the transmitting antenna. Similarly, the phase opposition of the receiving antenna serve to cancel spurious signals received from distant sources. The shield is used to shield the antennas from capacitively coupled spurious electrical signals. However, while the antenna disclosed in the aforementioned U.S. Pat. No. 4,251,808 does operate as an effective magnetic antenna for an electronic article surveillance system, the antenna is relatively difficult and costly to fabricate and tends to be bulky.